Theoretical Investigation Of The Ferroelectricity/ Piezoelectricity In Several Hydrogen-Bonded Systems

It is well-known that ferroelectric materials possess spontaneous electric polarizations that are switchable under an external electric field.Ferroelectric materials have been extensively studied because of their diverse applications in electro-optic,piezoelectric and nonvolatile memory devices.The first discovery of ferroelectricity in Rochelle salt that contains hydrogen-bonded tartrate ions opened the door to the study of ferroelectrics.Considering that the salt contains hydrogen-bonded tartrate ions,the relationship between hydrogen bond and ferroelectric is also revealed.In fact,that the extensive existence of ferroelectricity in H-bonding crystals can be attributed to the nature of hydrogen bonds: most H-bonds are dipolar and many H-bonded crystals lack a center of symmetry;moreover,hydrogen bonds are relatively weak bonds,which can be easy to break by applying an electric field.In addition,due to the feature of inversion symmetry breaking in ferroelectrics,H-bonding ferroelectrics are piezoelectric.Piezoelectric materials possess the capability of generating an electric potential in response to an applied mechanical stress or generating mechanical movement when subjected to an electric field.They have been used in many fields like sensors,transducers,nano-positioners,ultrasonic motors,imaging devices,etc..In this thesis,we use first-principles calculations plus Monte Carlo simulation to study the ferroelectricity,multiferroicity and piezoelectricity in several hydrogen-bonded systems.Firstly,we show the first-principles evidence of ferroelectricity with ultra-long iondisplacement in strong base: sodium and potassium hydroxides.Even a small number of proton vacancies can completely change the mode of proton-transfer from intra-unitcell to trans-unitcell,giving rise to multiferroic soliton with “mobile” magnetism and a tremendous polarization that can be orders of magnitude higher compared with most perovskite ferroelectrics.Their vertical polarizations of thin-film are robust against a depolarizing field,rendering various designs of two dimensional(2D)ferroelectric field effect transistors with nondestructive readout and ultrahigh on/off ratio via sensing the switchable metallic/insulating state.Then,we show ab initio designs of 0D/1D ferroelectrics/multiferroics based on functionalized transition-metal molecular sandwich nanowires(SNWs),which exhibit a series of intriguing properties.Some functional groups like –COOH will spontaneously form into three-fold helical hydrogen-bonded chains around SNWs with considerable and robust switchable polarizations.Two modes of ferroelectric switching are revealed: when the ends of SNWs are not hydrogen-bonded,the polarizations will be reversed via ligand reorientation that will reform the hydrogen-bonded chains and alter their helicity;when both ends are hydrogen-bonded,the polarizations can be reversed via proton-transfer while the helicity of chains will be constant.The combination of those two modes enables the system as the smallest proton conductor with a moderate migration barrier,which is lower compared with many prevalent proton-conductors for higher mobility while still large enough against thermal fluctuations at ambient conditions.This desirable feature can be utilized for constructing nanoscale artificial ionic synapses that may enable neuromorphic computing.In such design of synaptic transistors,protons migrating through those chains can be controlled and continuously change the conductance of MXene-based post-neuron for nonvolatile multilevel resistance,and the success of mimicking synaptic functions will make them promising in future high-density artificial neutral systems.Moreover,we propose a new approach to obtain ultra-high piezoelectric coefficients that can be infinitely large theoretically,where ferroelectrics with strain-sensitive Curie temperature are necessary.We show the first-principles plus Monte Carlo simulation evidence that many hydrogen-bonded ferroelectrics(e.g.organic Ph MDA)can be ideal candidates,which are also flexible and lead-free.Owing to the specific features of hydrogen bonding,their proton hopping barrier will drastically increase with prolonged proton transfer distance,while their hydrogen-bonded network can be easily compressed or stretched due to softness of hydrogen bonds.Their barriers as well as the Curie temperature can be approximately doubled upon a tensile strain as low as 2%.Their Curie temperature can be tuned exactly to room temperature by fixing a strain in one direction,and in another direction,an unprecedented ultra-high piezoelectric coefficient of 2058 p C/N can be obtained.This value is even underestimated and can be greatly enhanced when applying a smaller strain.Aside from sensors,they can also be utilized for converting either mechanical or thermal energies into electrical energies due to high pyroelectric coefficients.